This invention relates to curing accelerators for resin compositions containing an epoxide compound and a carboxylic acid anhydride.
It is known that carboxylic acid anhydrides are effective curing agents for epoxide compounds. The resultant resins contain carbonyloxy radicals ##STR1## that impart a desirable combination of high strength, thermal stability and high electrical resistance. The reaction between epoxide compound and acid anhydride is often relatively slow even at elevated temperatures. A number of acidic and basic compounds are therefore often employed as accelerators in epoxide-anhydride formulations. Phenols and tertiary amines are conventional accelerators, however both these classes of compounds are skin irritants and therefore undesirable, particularly in commercial scale operations employing large amounts of accelerators.
An objective of this invention is to provide accelerators that cure epoxide-anhydride compositions at relatively low temperatures without the hazards to operating personnel associated with use of prior art phenolic and amine accelerators. This objective can be achieved if the anhydride is first reacted with an organothiostannoic acid anhydride of the formula (RSnS.sub.1.5)n before being combined with the epoxide. In contrast to the monoorganotin compound, the reaction products are compatible with all classes of the epoxide compounds. When the epoxide compound is a relatively reactive one, such as a cycloaliphatic epoxide, an unreacted mixture containing the carboxylic acid anhydride and organotin compound is usually an effective curing agent.